The present invention relates to an apparatus and method for imaging time resolved fluorescence in biochemical and medical samples.
Fluorescence is emitted when a fluorophore interacts with an incident photon (excitation). Absorption of the photon causes an electron in the fluorophore to rise from its ground state to a higher energy level. After a time dependent on the fluorophore and its environment, the electron reverts to its original level, releasing a photon (fluorescence emission) with wavelength dependant upon the amount of energy that is released during reversion. A fluorophore may emit at single or multiple wavelengths (creating an emission spectrum), as electrons drop from various orbitals to their ground states. The emission spectrum is constant for each species of fluorophore.
Fluorescent labels typically are small organic dye molecules, such as fluorescein, Texas Red, or rhodamine, which can be readily conjugated to probe molecules, such as steptavidin. The fluorophores can be detected by illumination with light of an appropriate excitation frequency and the resultant spectral emissions can be detected by electro-optical sensors or by eye.
Methods of performing assays on fluorescent materials are well known in the art and are described in, e.g., Lakowicz, J. R., Principles of Fluorescence Spectroscopy, New York: Plenum Press (1983); Herman, B., Resonance Energy Transfer Microscopy, in: Fluorescence Microscopy of Living Cells in Culture, Part B, Methods in Cell Biology, vol. 30, ed. Taylor, D. L. and Wang, Y.-L., San Diego: Academic Press (1989), pp. 219-243; Turro, N. J., Modem Molecular Photochemistry, Menlo Park: Benjamin/Cummings Publishing Co, Inc. (1978), pp. 296-361 and the Molecular Probes Catalog (2001), OR, USA.
A fluorometer is an instrument that measures fluorescence. Fluorometers have three principal components: (a) light source for excitation, which is most typically a laser or broadband source; (b) filters and/or dispersive monochromators for selecting wavelength regions of interest, both in excitation and in emission; (c) a detector which converts the impinging fluorescence emission to an electrical signal. The present invention relates to fluorometers which measure fluorescence in biological samples, most specifically in biological samples used in drug discovery programs.
Some fluorometers (micro fluorometers) are constructed around confocal or noconfocal microscopes, and are designed for viewing fluorescently labeled cells or microvolumes of sample as discrete targets. There is a massive body of prior art relevant to such micro fluorometers. A subset body of this body of prior art relates to the use of micro fluorometers with microwell plates or wafers (e.g. Galbraith, et al., 1991; Rigler, 1995).
Other fluorometers (macro fluorometers) do not measure at cellular resolution but, rather, use low magnification optics to collect signal from each sample in a plurality of samples (as in U.S. Pat. No. 5,125,748 to Bjornson et al.; U.S. Pat. No. 5,340,747 to Eden), most typically wells are arranged within microwell plates (as in U.S. Pat. No. 6,071,748 to Modlin et al.; U.S. Pat. No. 5,670,113 to Akong et al.).
The means by which a plurality of samples is measured differs between a xe2x80x9cscanning fluorometerxe2x80x9d, a xe2x80x9cstepping fluorometerxe2x80x9d and an xe2x80x9carea imaging fluorometerxe2x80x9d.
In a scanning macro fluorometer (as disclosed in U.S. Pat. No. 5,672,880 to Kain), a plurality of samples is detected in serial fashion, by scanning an excitation beam over the sample and collecting each emission point serially. A scanning fluorometer can resolve each sample into multiple resolution points.
In a stepping macro fluorometer, the excitation beam and the detector (usually a photomultiplier tube or diode) move in stepwise fashion from sample to sample. The detector (or array of detectors) makes a unitary measurement from each sample, and does not discriminate multiple resolution points within each sample (as e.g. U.S. Pat. No. 5,589,351 to Harootunian; U.S. Pat. No. 5,670,113 to Akong et al.; U.S. Pat. No. 6,144,455 to Tuunanen et al; U.S. Pat. No. 6,127,133 to Akong et al.).
In an area imaging macro fluorometer, the detector is exposed to a plurality of samples in parallel (without scanning or stepping), and each sample is detected on a different portion of the detector area. By localizing different samples to different points on the detector, the area imaging fluorometer maintains the ability to discriminate discrete samples within a plurality of samples. The area imaging macro fluorometer may or may not discriminate multiple resolution points within each sample.
Area imaging macro fluorometers are well known (e.g. Haggart 1994; U.S. Pat. No. 6,140,653 to Che; U.S. Pat. No. 6,069,734 to Kawano et al.; many commercial systems). Most of these are suitable for flat samples such as electrophoresis gels. Some (e.g. U.S. Pat. No. 5,275,168 to Reintjes et al.; U.S. Pat. No. 5,309,912 to Knuttel) require highly specialized methods such as Raman spectroscopy or phase/amplitude modulation of illumination, and would not be suitable for either prompt or time resolved fluorescence intensity measurements as used with a plurality of biological samples. Others (U.S. Pat. No. 5,854,684 to Stabile et al.) mention the general use of imaging detectors to quantify light reflected from a plurality of samples, without describing specific embodiments within the illumination and optical detection systems that would allow effective use of the imaging detectors in macro fluorometry. Finally, others (WO96/05488 to McNeil et al.) disclose specific means to accomplish area imaging macro fluorometery, which means are not optimal for the purpose.
There is a need for high rates of sample throughput in drug discovery applications, wherein fluorescent specimens are usually arrayed within a plurality of wells and high throughput is required. While an area imaging fluorometer offers known advantages in its ability to quantify large numbers of samples in parallel, thereby achieving high throughput, specific embodiments appropriate to this purpose are required if the instrument is to be both high in throughput and high in sensitivity.
It is obvious, in light of the art of area imaging macro fluorometers, to position a CCD camera so as to image a specimen, deliver fluorescence excitation by known means, and collect emitted light using a lens in a known fashion. However, imaging fluorometers of the kind disclosed by McNeil et al. (WO96/05488) do not achieve equivalence in sensitivity with non-imaging fluorometers, and suffer from errors in signal detection. For example, they use standard lens systems which do not contain mechanisms so calculated as to minimize reflections and other forms of spurious signal, suffer from parallax error, and usually require that microwell plates be of the kind with a transparent bottom, through which the samples are read. In contrast, non-imaging fluorometers are highly sensitive, have no parallax error, are able to read from top or bottom of a plate, and do not require expensive transparent-bottom plates. Potential users of area imaging fluorometers in drug screening have not adopted widely any prior art system, because of factors such as those detailed above.
Challenges in applying area imaging fluorometry to drug screening arise, in the main, from four factors:
a) The wells form apertures which interact with the angle of incidence of the illumination. Wells which receive illumination at greater angles are less well illuminated than wells which receive more direct illumination. As most area imaging fluorometers deliver illumination from a region lateral to the collection lens (e.g. Neri et al., 1996), they can be successful with flat specimens but fail to properly illuminate deep wells. In practice, it is not uncommon to observe variations in excess of 300% in the delivery of light to wells lying at different positions in a microwell plate unless illumination delivery occurs only through the bottom of the microwell plate (as in U.S. Pat. No. 5,355,215 to Schroeder and WO96/05488 to McNeil et al.).
b) The wells are very dim. A plurality of wells contained within a standard microwell plate occupies a spatial extent of approximately 96 cm sq. If a macro area imaging fluorometer is to be capable of imaging all wells in parallel, all the wells must be illuminated at the same time. Therefore, the amount of illumination delivered to any one well (as described by an inverse geometric relationship between illuminated area and intensity/unit area) is very low and the amount of fluorescence emitted from any one well also tends to be small and a sensitive detector is required. However sensitive the detector, it is important that all wells receive the illuminating beam as efficiently as possible. If illumination fails to reach the contents of some wells, even the most sensitive detector will be unable to extract a usable signal.
c) There is parallax error. Deep wells located at different positions within a microwell plate are imaged at different angles by a standard lens. This well known parallax error has a major effect upon the accuracy of the light collection optic, particularly when the optic images the plurality of wells from the top.
d) The illumination rays interact with the detection optics. Optics for illumination delivery to an entire well plate, and for simultaneous collection of fluorescence emission from the plurality of wells in the plate must have special properties. For example, the most efficient illumination method is from within the collecting lens (epi-illumination), but a macro epi-illuminating lens is subject to reflections and other non-obvious sources of spurious signal.
A digital system imaging system for assays in well plates, gels and blots is disclosed in U.S. patent application Ser Nos. 09/240,649 and 09/477,444 by Ramm et al. This system meets the need for high throughput area imaging fluorometry, and achieves a technical advance in that it overcomes the problems raised in points a-d, above. Most specifically, the disclosed system:
a) minimally interacts with the well apertures so as to illuminate a plurality of wells in an efficient and even fashion, whether from above or below;
b) by illuminating in an efficient manner, allows better sensitivity in area imaging fluorometry;
c) uses a telecentric optic to avoid parallax error;
d) contains optics and components calculated to minimize sources of background signal arising in macro area imaging fluorometry (e.g. minimizes internal reflections from emission filters mounted within the collection lens).
The disclosed system has been reduced to practice and widely used for area imaging of luminescence and prompt fluorescence signals. Means to use the system for time resolved fluorescence imaging are disclosed in the cited patent applications, but without detailed description of the embodiments for this purpose. The present invention discloses preferred embodiments of an area imaging fluorometer for time resolved assays in well plates, gels and blots.
Prompt fluorescence emission is defined by the delivery of excitation illumination at the same time as emitted fluorescence is collected. Challenges in prompt fluorescence include:
very small alterations in emission must be detected while the system is flooded with excitation light;
emission is much lower in intensity than excitation, and it is difficult to filter out all of the excitation so as to leave uncorrupted emission;
the sample medium (e.g. solvent, cells containing labeled compound), optics (e.g. Beverloo et al. 1990, 1992) and sample carrier (well plate, cuvette) can emit spectral components similar to those of the fluorophore of interest.
Collectively, any excitation light or sample medium contributions that reach the detector are termed xe2x80x9cbackgroundxe2x80x9d. An emission filter is placed between the sample and the detector so as to block as much as possible of the background signal from reaching the detector (as disclosed in International Patent Application Publication WO99/08233 to Ramm et al.). However, the emission filter cannot remove all of the background and the remaining leakage of background onto the detector results in a degradation in detection sensitivity. That is, the sensitivity of fluorometers tends to be limited by the presence of high levels of background signal.
Time resolved fluorescence (TRF) is a well-known technique (e.g. U.S. Pat. No. 4,565,790 to Hemilla et al., U.S. Pat. No. 4,808,541 to Mikola et al., U.S. Pat. No. 5,736,410 to Zarling et al.) used to reduce background and increase detection sensitivity. In TRF, a light source is used to excite the sample and is then quickly extinguished or occluded. During the period of excitation, the detector is turned off. Being off, it is not subject to background contributions arising from the period of excitation (e.g. leakage of the excitation light through the emission filter).
Most background contributions are absent or decay rapidly (e.g. solvent fluorescence) after termination of excitation. In contrast, fluorophores do not cease their emission immediately. Rather, there is a period (ranging from picoseconds to milliseconds) during which the fluorophore continues to emit after the excitation is terminated. During this period, the detector is turned on to observe relevant fluorescence signal with minimal background. This delayed timing of the detection period is the key principle of TRF.
To enable TRF detection, the detector must be turned on and off within a controlled time period. The detection period starts some time after termination of excitation (after a delay time) and lasts for a period of time (the acquisition time). TRF is most often used with fluorophores based on lanthanides (Eu, Tb, etc.), that have decay times of the order of 100 microseconds to 2 milliseconds, much longer than the typical decay times of the background sources. Typically, the delay time is on the order of tens of microseconds, and the acquisition time is tens to hundreds of microseconds.
During the excitation period and the delay time, the detector is occluded or turned off (hereinafter referred to as gated off). During the acquisition time, the detector is exposed to emissions from the sample (hereinafter referred to as gated on). It is unlikely that any single cycle of gating off and gating on will generate a reliable signal on the detector, so the cycle is repeated until sufficient signal has accumulated.
Delayed fluorescence emission is much less intense than prompt fluorescence emission, and area imaging systems must work with very low levels of emission/unit area. Therefore, an area imaging fluorometer is more subject to the loss of delayed emission in the noise floor of the detector than is another type of fluorometer. Hence, it is desirable for a TRF system to have a geometry which results in high optical throughput (etendue) and low transmission losses. Challenges arise in optimizing a system to achieve this combination of rapid gating and high etendue.
Both the excitation light and the detector must be gated. Gating of excitation with mechanical choppers and/or by using pulse lasers (e.g. Gadella and Jovin, 1995) is widely known, as is use of flash lamps which decay quickly enough that their contribution to the delayed acquisition is minimal (as disclosed in WO 99/08233 to Ramm et al.; EP 0987540A2 to Vaisala et al.; Vereb et al. 1998).
There are three methods in common use for the gating of the emission pathway in TRF:
(a) A shutter wheel is rotated rapidly to produce microsecond-scale gating of the emitted light. This approach has been applied in TRF microscopy (Seveus et al., 1992) to reject the signal from prompt fluorescence and thereby reduce autofluorescence. A similar method is disclosed in EP 0626575A1 to Vaisala et al., and in scanning detection systems (U.S. Pat. No. 5,780,857 to Harju, et al). On a microscope or a system which detects one sample at a time, a single fast shutter is viable because the etendue is very small. In contrast, the etendue of macro imaging systems is large, about two orders of magnitude greater than that of a microscope. A macro imaging system would require a large mechanical shutter mechanism, which has a relatively gradual transition from open to closed.
(b) A multicomponent mechanical shutter, using fast rotating shutter wheels has openings synchronized in such a way to expose small sections of the detector at a time. This arrangement, (as disclosed in EP 0987540A2 to Vaisala et al.) is faster than a single wheel, but is complex, composed of rapidly moving parts, and has poor etendue.
(c) The detector is gated, electrically, and also converts emission photons into electrons. In this method a photocathode is placed into the optical path. The photocathode can be turned on and off at short intervals to accommodate the timing requirements of TRF. However, this type of device has low quantum efficiency and could not be used to image dim fluorescence emission onto an imaging detector. Therefore, an accelerating electrical field is applied to increase the energy of the photoelectrons prior to the target and, in many cases the accelerated electrons are guided through a microchannel plate, where they initiate the emission of many more accelerated electrons. Thus, the electrically gated detector usually incorporates a light amplifier, which amplifies the electron stream prior to the target.
A macro TRF imaging system using an electrically gated detector, with a photocathode and microchannel plate amplification has been disclosed (WO 99/08233, Ramm et al; Ramm, 1999). The use of an amplified photocathode between the lens and the detector has inherent disadvantages:
(a) Most typically, TRF is emitted at long wavelengths. This is particularly true with donor-acceptor pairs in fluorescence resonance energy transfer (FRET), which tend to emit in the red portion of the spectrum. To detect these types of signals, the bandgap of the photocathode needs to be sufficiently low to allow a long wavelength photon to generate a photoelectron. However, low bandgap materials are prone to generate thermal signals and produce an instrumental background that can be indistinguishable from the signal to be detected. Mildly cooling the photocathode (as reduced to practice in commercially available devices) does not eliminate thermal background. Deep cooling of the photocathode to temperatures sufficiently low to greatly reduce thermal background presents significant technological challenges and is not in common use.
(b) The camera component of an intensified system produces noise, particularly thermal background. To overcome this, the CCD can be cooled to reduce thermal background (as reduced to practice in a number of commercial devices). Difficulties reside in cooling a fiber-coupled CCD that is absorbing heat load from an intensifier, and in maintaining a bond between the warm fiber coupling and the cold CCD.
(c) The QE of the gating/amplification component is low, typically on the order of 10% in blue-green wavelengths, and as low as 1% in the red. Therefore, most of the signal incident to the amplified gating system is lost, and use of the system with time resolved fluors emitting in the red is problematic.
(d) The amplification process does not produce a constant number of electrons per incident photoelectron. This gain noise is, typically, overcome by setting a minimum threshold for detected events, and counting only events which have a high probability of being valid. In this way, the amplified device is operated as a counter, instead of as an integrating imager. While highly sensitive in this counting mode, the amplified device is also very inefficient and long periods of detection are required to accumulate enough suprathreshold events to create an image.
There are other disadvantages, including fragility of the light amplifier (it is easily destroyed by bright lights) and poor modulation transfer in the amplified images. Further, bright areas of the specimen will tend to influence areas around them and, therefore, imaging of microwell plates is highly subject to crosstalk between wells. In sum, prior art systems using photocathode gating and amplification all exhibit shortcomings in the field of the present invention.
It is possible to construct a macro area imaging fluorometer for TRF, in which the gating and amplification functions are inherent to the detector. For example, it has been determined that an electron bombarded CCD (EBCCD) can be implemented within a system of the configuration disclosed by Ramm et al. (WO 99/08233). It is envisaged that other integrally amplified detectors, such as the electron amplifying CCD (as reduced to practice by Marconi Applied Technologies of the United Kingdom), would also be applicable within a system of the disclosed configuration. In comparison to intensified CCD cameras, it is an advantage of the EBCCD (and perhaps other integrally gated detectors) that it shows very little crosstalk in imaging microwell plates, while it remains sensitive enough to detect faint delayed fluorescence signals.
Almost any type of fast shutter could be envisaged as a gate for a TRF system. TRF with a macro epi-illuminating system using a liquid crystal, leaf or other shutter mounted within the optical path after the specimen but prior to the detector has been disclosed (WO 99/08233, Ramm et al.). In a preferred embodiment, the present invention incorporates such a shutter implemented within a system which is so optimized as to allow its use.
While automation of the process by which the present invention is applied is not specifically described, it can be envisaged that the present invention can be used with systems and methods that utilize automated and integratable workstations for identifying modulators, pathways, chemicals having useful activity and other methods described herein. Such systems are described generally in the art (see, U.S. Pat. No. 4,000,976 to Kramer et al. (issued Jan. 4, 1977), U.S. Pat. No. 5,104,621 to Pfost et al. (issued Apr. 14, 1992), U.S. Pat. No. 5,125,748 to Bjornson et al. (issued Jun. 30, 1992), U.S. Pat. No. 5,139,744 to Kowalski (issued Aug. 18, 1992), U.S. Pat. No. 5,206,568 Bjornson et al. (issued Apr. 27, 1993), U.S. Pat. No. 5,350,564 to Mazza et al. (Sep. 27, 1994), U.S. Pat. No. 5,589,351 to Harootunian (issued Dec. 31, 1996), and PCT Application Nos: WO 93/20612 to Baxter Deutschland GMBH (published Oct. 14, 1993), WO 96/05488 to McNeil et al. (published Feb. 22, 1996) and WO 93/13423 to Agong et al. (published Jul. 8, 1993).
In accordance with the present invention, an area imaging system for time resolved fluorometry of samples in well plates, gels, and blots incorporates a fast-acting electronic emission shutter implemented within a strobe-illuminated system of large etendue, coupled to a camera of high sensitivity (preferably as disclosed in WO 99/08233 to Ramm et al.). Preferably, but not necessarily, the emission shutter of the system is based upon ferroelectric crystal (FLC) technology. The FLC shutter is a ferroelectric-based device composed of an input polarizer, a ferroelectric material that changes the rotation of polarization depending on the electric field, and a polarizing analyzer inserted in the emission path of array imaging systems, between the sample and a solid state array detector.
It is a property of the system of the present invention that the optical/detection components containing the FLC shutter are so sensitive that transmission loss through the electronic shutter is accommodated without requiring a light amplification stage. In a preferred configuration, the quantum efficiency of the detector is high and the etendue of the optics is large. When used within such a system, the shutter of the present invention transmits enough delayed fluorescence to yield measurements comparable to those obtained from non-imaging time resolved fluorometers, and exceeds the sensitivity of systems using amplified photocathodes without the disadvantages (crosstalk, fragility, etc.) of such systems.
One skilled in the art will appreciate that a fast electronic shutter would be useful as a gating mechanism for TRF. However, most fast shutters like Kerr cells, opto-acoustical modulators or electro-optical modulators, have limited etendue restricting their applications to systems with small etendue like microscopes. It is an advantage of the electronic shutter of the present invention that it has a large aperture and takes advantage of the etendue of the system of the present invention.
The system of the present invention could be equipped with a liquid crystal-based shutter. While this type of shutter can have a sufficient aperture to be used with the large etendue of the present system, it has a poor ratio of the transmissive state to the blocking state (defined as contrast ratio). Most typically, the contrast ratio of a liquid crystal shutter is on the order of  less than 200:1, as compared to the  greater than 400:1 observed with an FLC shutter. The liquid crystal-based shutter is also not sufficiently fast for TRF imaging with many commonly used fluors.
Preferably, the system of the present invention incorporates a ferroelectric crystal-based (FLC) shutter. It is an advantage of the system of the present invention that the FLC shutter can transit from gated on to gated off within less than 100 microseconds, and is suitable for delayed fluorescence imaging of commonly used lanthanide, europium and terbium fluors.
The disadvantage of FLCs, as disclosed in previous attempts to use FLC shutters in TRF microscopy, has been poor rejection of the prompt fluorescence due to an inadequate contrast ratio (Verwoerd et al., 1994; Shotten, 1995). This poor contrast ratio has forced the use of two FLC shutters in series, and the resulting transmission loss has been so high (Vereb et al., 1998) that use in a macro imaging fluorometer would be impossible. In the present invention, the FLC is placed within the optics, in a position to yield the best possible contrast ratio. It is a property of the system of the present invention that the optics are so constructed that the fast shutter is positioned in the optical path of the emitted fluorescence in such a manner that the incidence angles with respect to the normal onto the shutter do not exceed 20xc2x0. This positioning achieves the best possible contrast ratio from the shutter, and minimizes the effects of said shutter upon the wavelength characteristics of the transmitted light.
In a preferred embodiment of the present invention, leakage through the FLC shutter is corrected by first taking an image of prompt fluorescence with the shutter closed (the leakage image). This image is read out from the CCD camera, prior to acquisition of the delayed fluorescence image. Then, the leakage image is subtracted from the delayed fluorescence image to leave an image of delayed fluorescence without background added by leakage through the FLC shutter.
It is an advantage of the illumination system and electronic shutter of the present invention that they can be adapted to use in existing macro area imaging fluorometers, without requiring major modifications to said devices (e.g. as disclosed in WO 99/08233, Ramm et al.). Rather, the illumination system of the present invention can be mounted within an existing macro area imaging fluorometer, and the shutter can also be mounted within said device without major modification.
It is an advantage of the present invention that the shutter is easily inserted or removed from a macro area imaging fluorometer, allowing said fluorometer to be used for a very broad variety of applications without interference from said shutter.
It is an advantage of the system of the present invention that it can be used with systems and methods that utilize automated and integratable workstations for identifying modulators, pathways, chemicals having useful activity and other methods described herein. Such systems are described generally in the art (see, U.S. Pat. No. 4,000,976 to Kramer et al. (issued Jan. 4, 1977), U.S. Pat. No. 5,104,621 to Pfost et al. (issued Apr. 14, 1992), U.S. Pat. No. 5,125,748 to Bjornson et al. (issued Jun. 30, 1992), U.S. Pat. No. 5,139,744 to Kowalski (issued Aug. 18, 1992), U.S. Pat. No. 5,206,568 Bjornson et al. (issued Apr. 27, 1993), U.S. Pat. No. 5,350,564 to Mazza et al. (Sep. 27, 1994), U.S. Pat. No. 5,589,351 to Harootunian (issued Dec. 31, 1996), and PCT Application Nos: WO 93/20612 to Baxter Deutschland GMBH (published Oct. 14, 1993), WO 96/05488 to McNeil et al. (published Feb. 22, 1996) and WO 93/13423 to Akong et al. (published Jul. 8, 1993).
While the present invention is directed at time resolved macro fluorescence area imaging, it is to be understood that one skilled in the art could use the polarization capabilities of the present invention, as embodied in the FLC shutter or other shutter incorporating polarizing optics, to construct a polarization macro fluorescence area imaging system. In such system, the excitation light is polarized by first passing through a FLC shutter or some other polarizing device. The emitted light is passed through a polarizer mounted within the emission path. Either polarizer may be rotated with respect to the other, and the system is of such high sensitivity that it achieves polarization measurement comparable to that of non-area imaging devices.